Because LCD devices have the advantages of portability, low power consumption, and low radiation, they have been widely used in various portable information products such as notebooks, personal digital assistants (PDAs), video cameras, and the like. Furthermore, LCD devices are considered by many to have the potential to completely replace cathode ray tube (CRT) monitors and televisions.
Referring to FIG. 4, a typical LCD device 10 includes a gate driving circuit 11, a data driving circuit 12, and a liquid crystal panel 101. The gate driving circuit 11 is configured for providing a plurality of scanning signals to the liquid crystal panel 101. The data driving circuit 12 is configured for receiving image data from an external source, and providing a plurality of gray scale voltages to the liquid crystal panel 101 accordingly.
The liquid crystal panel 101 includes a plurality of parallel scanning lines 13, a plurality of parallel data lines 14, a plurality of thin film transistors (TFTs) 15, a plurality of sub-pixel electrodes 151, and a plurality of common electrodes 152. The gate lines 13 and the data lines 14 cross each other and cooperatively define a plurality of sub-pixels 16 arranged in a matrix. The liquid crystal panel 101 also includes a layer of liquid crystal spanning the entire matrix. The liquid crystal contains liquid crystal molecules.
The TFTs 15 are arranged in a matrix respectively corresponding to the sub-pixels 16. Each TFT 15 includes a gate electrode (not labeled) connected to the corresponding gate line 13, a source electrode (not labeled) connected to the corresponding data line 14, and a drain electrode (not labeled) connected to a corresponding sub-pixel electrode 151. Each sub-pixel electrode 151 is opposite to a corresponding common electrode 152. All the common electrodes 152 are substantially connected to a common voltage source (not labeled), which has a predetermined common voltage applied thereto.
The sub-pixels 16 includes a plurality of red sub-pixels (R), a plurality of green sub-pixels (G), and a plurality of blue sub-pixels (B), which are arranged in a pattern of repeating RGB sequences in each row of the matrix.
Generally, when the LCD device 10 displays images, the common electrode 152 has the predetermined common voltage applied thereto, and the sub-pixel electrode 151 has a gray scale voltage applied thereto. Thus, an electric field is generated in the area of the liquid crystal molecules at each sub-pixel 16. A transmittance of light passing through the liquid crystal molecules is adjusted by controlling the strength of the electric field. Thereby, the desired transmittances of light obtained at all the sub-pixels 16 cooperatively produces an image viewed by a user of the LCD device 10.
Referring to FIG. 5, this is a schematic diagram of part of a testing image 30 for the LCD device 10. At each sub-pixel 16, the liquid crystal molecules in the electrical field are twisted such that light rays are allowed to pass through the sub-pixel 16. When the gray scale voltage is greater than the common voltage, the direction of the electrical field is from the sub-pixel electrode 151 to the common electrode 152, and the sub-pixel 16 has a positive polarity (+). Conversely, when the gray scale voltage is less than the common voltage, the direction of the electrical field is from the common electrode 152 to the sub-pixel electrode 151, and the sub-pixel 16 has a negative polarity (−). Moreover, when absolute values of the gray scale voltages applied to the sub-pixel electrodes 151 of two sub-pixels 16 are the same, and the gray scale voltages only differ in polarity, the gray scales of the two sub-pixels 16 are assumed to be the same. The liquid crystal panel 10 is a normal white mode panel. That is, the greater the gray scale voltage applied, the less the amount of light rays that can pass through the corresponding sub-pixel 16. When the gray scale voltage is great enough, the light rays cannot pass through the corresponding sub-pixel 16.
As shown in FIG. 5, each small square represents a sub-pixel 16. Each sub-pixel 16 has a polarity different from the polarity of the two adjacent sub-pixels 16 in the same row, and different from the polarity of the two adjacent sub-pixels 16 in the same column. Furthermore, squares that are not hatched represent sub-pixels 16 that have no light passing therethrough. When the testing image 30 is displayed, in the first row of sub-pixels 16, there are eight sub-pixels 16 having positive polarity that no light rays can pass through and ten sub-pixels 16 having negative polarity that no light rays can pass through. Therefore, among the sub-pixels 16 in the first row that having no light rays passing through, an amount of the sub-pixels 16 having positive polarity is less than an amount of the sub-pixels 16 having negative polarity. When the gray scale voltages are applied to the sub-pixels 16 in the first row which have no light rays passing therethrough, the gray scale voltages applied to the corresponding sub-pixel electrodes 151 are liable to drag down the common voltage of the first row of sub-pixels 16 due to a coupling effect. Thus, actual common voltages of the common electrodes 152 corresponding to the first row of sub-pixels 16 are slightly less than the predetermined common voltage.
Accordingly, the actual common voltages corresponding to other common electrodes 152 are dragged down or up immediately the gray scale voltages are applied to the corresponding sub-pixel electrodes 151, thereby generating common voltage variations.
Because the actual common voltages are not equal to the predetermined common voltage, the testing image 30 may be impaired by a so-called crosstalk phenomenon. That is, the testing image 30 may be visibly flawed. Further, when ordinary images are displayed, crosstalk may also occur when actual common voltages are not equal to the predetermined common voltage.
What is needed, therefore, is an LCD device that can overcome the above-described deficiencies.